1. Field of the Invention
Large rotating rotors, either solid or hollow, are used in many applications both in the utility industry and the petro-chemical industry. These large rotating shafts are used in equipment which often may operate at elevated temperatures. Many of these large rotating shafts were originally produced from ASTM A-470 materials which are usually identified as Cr--Mo--V, Ni--Mo--V, or Ni--Cr--Mo--V low alloy steel materials which have been heat treated to differing strength levels for individual applications. Their use in turbines exposes these alloys to high pressures and temperatures, cycling temperature differentials, vibrations, low cycle fatigue, creep-fatigue, and possibly high cycle fatigue problems during the normal course of rotor service operation. In addition to high pressures and high temperature wherein shaft temperatures usually exceed 500.degree. F., certain high stress blade attachment sections thereof operate on a continuous basis at temperatures at or exceeding 950.degree. F. These large rotating shafts are also exposed to the hostile environments of boiler steam, as well as being subjected to vibration/misalignment and other mechanical stresses. Also, these rotor shafts, or simply "rotors," typically contain a myriad of small minors defects resulting from the original casting and forging processes. These combinations of temperatures, stresses and minor defects can influence the cracking or deformation in such rotors.
The Cr--Mo--V, Ni--Mo--V, and Ni--Cr--Mo--V low alloy steels, commonly used in these rotors, have generally provided fairly good high temperature fatigue and creep properties. However, owing to limitations in the original manufacturing process, forty year old design limitations, instrumentation limitations, and operation limitations, the higher stress areas of some of these rotors are the first to distort or crack.
More important, the sections of these rotors which operate at temperatures over 800.degree. F. for times often exceeding 200,000 hours operate in the creep range. Creep is a phenomenon wherein permanent deformation occurs over a long period of time. Metallurgically, creep results in grain boundary separation, microcracking, visible cracking and finally component failure for low alloy steels. This problem is exacerbated by the thermal cycling that occurs during normal operation and which causes a complex interaction of creep and fatigue. Current utility trends to practice load-following causes additional cycling of their generating units which also increases the susceptibility to have cracking or deformation occur in the rotors thereof sooner than would have been expected from the original design operating conditions. Even when operating within the design envelope, the average age of many electric utility rotors is now approaching 40 years. Accordingly, these rotors are now operating essentially beyond their originally intended design life. Nonetheless, the cost to replace them would be extremely prohibitive as a normal method of operation in today's competitive market.
Accordingly, a real need has long existed for high quality weld repair methods and means which include a complete reanalysis of the component operating conditions to optimize the room temperature and operating temperature mechanical properties of the rotor base metal, the HAZ, the proper location of the so-called fusion line, and techniques for deposit of the buttering layer on the prepared rotor surface juxtaposed the fusion line as well as the metal buildup thereover and the alloy used therefore.
2. Description of the Prior Art
Large rotating shafts made from ASTM A-470 type alloys are considered difficult to weld. This difficulty is attributed mainly to their rather high carbon content, which is inherently necessary in order to produce adequate strength, and the unintended high levels of residual elements which were characteristic of steel making practices in the 1950's and 1960's. In spite of the welding difficulties the high cost associated with the alternative of replacement of these critical components has led to the development of many different, albeit, less than totally successful weld procedures for effecting such repairs.
For ease of understanding and convenience to the reader the following description of numerous prior art references and teachings has been organized, as far as was possible, into the sequence and arrangement of Embodiments One through Five discussed, supra.
Embodiment One--Repair of Worn Rotor Surfaces. One of the earlier patents dealing with the repair of worn surfaces on rotor body is found in U.S. Pat. No. 4,710,103, Faber et al., Dec. 1, 1987, wherein is disclosed a rotor body having an outer layer deposited thereon by means of one or more welding runs and wherein that outer layer is subsequently machined into, for example, steeples for purposes of receiving blades, which blades are subsequently inserted thereinto. Although the disclosure in '103, supra is broad in scope, essentially the same disclosure was published at an earlier date, to wit, in April 1984 Amos, D. R. and Clark, R. E., Reparation de soudures pour rotors de turbines, IIW Doc. XII-844-84, April 1984 (Technical Welding Institute, Abington Hall, Abington, Cambridge, UK)!.
In a somewhat later disclosure Clark et al. in U.S. Pat. Nos. 4,897,519, Jan. 30, 1990, and 4,958,431, Sep. 25, 1990, disclose a method for repairing worn surfaces of Cr--Mo--V steam turbine components. A critical feature disclosed and claimed by Clark et al. is the procedure of depositing the ferrous alloy on worn surfaces in a multipass buildup comprising at least two spaced apart weld beads. As the rotor is turned beneath the welding torch for deposit of weld metal thereon and in a manner transverse to the rotor body the first weld bead is laid down in a first position and the second pass over the rotor body, instead of being laid down juxtaposed to the first weld bead is spaced apart essentially a substantial distance therefrom and thereafter the spaced apart procedure is continued presumably to maximize the heat sink characteristic of the rotor body. Although this spaced apart procedure might be desirable on very small rotors wherein very limited heat sink characteristics are available, it has been found that for large rotors of the type herein described and disclosed ranging from about 20 inches or larger in diameter, the heat sink available by such a large mass of metal renders this particular procedure of Clark et al. both unnecessary and inadvisable in that such a spaced apart arrangement of beads has been found to contribute to welding defects brought about due to lack of proper fusion including with the side wall of the adjacent bead.
In another procedure disclosed again by Clark et al. in U.S. Pat. Nos. 4,903,888, Feb. 27, 1990, and 4,940,390, Jul. 10, 1990, therein is taught the repair of worn surfaces on relatively low alloy, low chromium rotors wherein is disclosed a specialized tempering procedure effected on the HAZ, which HAZ in the region of the outer periphery of the rotor surface at and just below the fusion line formed by the first weldment layer deposited thereon. In this procedure of Clark et al., the tempering of the HAZ is effected by a reheating thereof to relieve internal stress and reduce hardness as well as overcoming the coarse grain structure effected in the HAZ by the first layer of weldments juxtaposed said fusion line. The specific procedure taught by Clark et al., is to lay down their second layer of weldment material under welding conditions wherein a higher heat is used than was used to apply or deposit said first layer, said higher heat utilized during depositing of said second layer giving rise to tempering at least a portion of said HAZ in the rotor at or near its prepared surface, to wit, the fusion line. Clark et al., further teach that this application of higher heat in effecting the deposit of their second layer of weldment is further utilized during the deposit of subsequent weldments for laying down or depositing the third, fourth, and fifth layers so as to continue utilizing the higher heat input for effecting the tempering of the HAZ. As may be seen from the instant teachings, infra, the advisability of utilizing such higher subsequent welding heat inputs is questionable at best, and indeed leads those skilled in the art in a direction just opposite to the discoveries comprising the instant invention.
Embodiment Two--Repair of Severely Cracked or Otherwise Damaged Rotors. A number of patents teach numerous methods for repairing steam turbine generator rotors wherein one end of each of two rotor segments are machined for mating thereof, or one end of a rotor segment is machined for mating with the end of a replacement section and the two resulting end faces are welded to one another by any number of procedures.
One such procedure is shown by Clark et al., U.S. Pat. No. 4,633,554, Jan. 6, 1987, wherein they further provide for the procedure of boring out the center of the attached rotor segment and a portion of the original rotor segment, wherein the interface therein caused by the weldment can be inspected deep within the core of the rotor without substantially destroying the strength characteristics thereof.
In another variation of this procedure, this time by Amos et al., U.S. Pat. No. 5,172,475, Dec. 22, 1992, a rotor is severed in close proximity to a crack whereupon a weld buildup portion is deposited onto the surface adjacent the core of each of the two pieces with subsequent machining of one such buildup portion into a first mating connection and the other buildup portion machined into a second mating connection for engagement of the pieces to ensure proper alignment of the engaged rotor pieces prior to the subsequent welding thereof. Amos et al. claim that their procedure of using the two original pieces with weld buildup made on each such piece and followed by subsequent machining can substantially cut the down time of the component as compared to joining on a new replacement rotor section with one of the original rotor segments. This is understandable since a new segment would have to be specially forged and constructed to meet the physical requirements of the desired finished product.
In the instance of repairing a rotor having a rather severe crack therein Galanes in U.S. Pat. No. 5,280,849, teaches forming a narrow groove in the rotor in a fashion to substantially remove the crack, i.e., the groove is deep enough and wide enough to cut out material juxtaposed the crack, whereupon the resulting groove, after preheating, is filled with his specialized welding filler metal and later heat soaked to result in his repair.
In still another disclosure of Clark et al., in U.S. Pat. No. 4,962,586, Oct. 16, 1990, they teach the joining of two rotor segments composed of different alloys, to wit, one being of a high temperature alloy and the other being of a low temperature alloy. In the practice of their procedure they selectively clad the machined surface of the rotor segment composed of a Cr--Mo--V alloy and then, after machining for alignment with the machined end of a rotor segment comprised of a Ni--Cr--Mo--V alloy, they fill the gap therebetween with a weld filler material to result in a composite comprised of four different metal alloys with which they claim to be able to bridge the gap between their high temperature alloy rotor segment and their low temperature alloy rotor segment. The procedure in this disclosure of Clark et al. of utilizing a cladding on at least one of the two machined end pieces of the rotor is of particular interest in that the instant invention in several embodiments thereof incorporate a buttering procedure wherein either one or both of the juxtaposed end faces of aligned rotor segments are first clad or buttered with specially applied weldments prior to the use of weld filler material therebetween, which in the instant invention is the same alloy composition as the buttering material.
In a still later issued patent to Galanes, U.S. Pat. No. 5,348,212, Sep. 20, 1994, there is shown a variation of the method of his earlier disclosure wherein the same welding Cr--Mo--V filler type alloy is utilized, but instead of being deposited in a narrow groove which has first been machined out to remove a crack in the rotor, it rather is used to fill a gap formed by mating machine surfaces of two rotor segments with the added feature of forming or attaching a pilot stub to the end surface of his first shaft segment and forming a pilot hole in the opposing end surface of his second shaft segment whereby positioning the pilot stub inside the pilot hole provides for more desirable alignment of the two rotor segments.
Embodiment Three--Utilization of Special Cover Gases and Shrouding Above the Weld Puddle During Hot Wire Weldment Buildup. As noted both supra and as will be apparent from the discussion infra, after deposit of the buttering layer onto the prepared rotor surface in Embodiment One supra or the end faces of one or both of the rotor segments in Embodiment Two, supra, the positioning and deposit of most of the total weld buildup metal, either for the later machining into portions of discs including steeples or for the filling of the gap between the mating end faces of two rotor segments is accomplished by hot wire welding as opposed to the cold wire procedure necessary for the buttering operation. Due to the substantially higher operating conditions during such hot wire weld buildup, the resulting weld puddle is substantially bigger or greater in volume and particularly in the case of the application to worn surfaces of rotors extends several inches from the point of welding transversely along the periphery of the rotor or rotor wheel. During the development of the instant invention, it was discovered that the length of the weld puddle be protected from the oxygen in the atmosphere until said weld solidifies, and further until it cools from its initial solidification temperature of perhaps about 2800.degree. F. to a temperature as low as about 800.degree. F. in order to minimize crust formation thereon which crust can adversely interfere with the next pass of weldment by undesirable inclusions therein of foreign elements which may or may not be "floated" in the subsequent pass of a weld puddle thereover. In addition, it has now been found that although the cold wire weldments for effecting the buttering layers, supra, are preferably flooded with argon to protect their respective weld puddles from unwanted and undesirable contact with oxygen in the atmosphere, the flooding of the hot wire welding puddle requires a special mixture of argon with helium in order to produce the desired characteristics necessary in the resulting weld. For instance, it is now known that the addition of helium to the argon gas cover over the weld puddle in the right proportions effects the aspect ratio of the weld nugget (bead width to penetration depth) so that the weld bead is effectively widened and wherein improved penetration of the puddle into the previous weldment minimizes the occurrence of lack-of-fusion defects. Although gas flooding or as herein termed "covering" of the weld puddle is inherently effected by the very nature of arrangements such as gas tungsten arc welding. However, the instant invention comprises further refinements comprising the instant invention whereby a shroud is utilized for further containing the cover gas over substantially all of the weld puddle and metal contiguous thereto and further wherein cover gas added at the situs of the arc comprises a mixture of helium with argon and, further wherein a unique arrangement of cover gases introduced in a particular sequence over the solidifying weld puddle and subsequently cooling weld bead.
Embodiment Four--Composition of the New Weld Wire Alloy. As noted above, the new weld wire alloy composition which has been developed for the practice of the instant invention is used in both the cold wire buttering procedure for the buildup of the first four or five layers onto the first prepared rotor surfaces as well as for the hot wire procedure which is subsequently used to lay down and deposit the bulk of the weld wire buildup, be it over said prepared surfaces or within narrow grooves or used to fill gaps between opposing rotor end faces of two or more rotor segments.
The new composition comprising the instant invention weld wire alloy is based in part on the 9Cr alloy originally developed by the Department of Energy at the Oak Ridge National Laboratory, circa 1975, which alloy was at that time designed for high temperature creep strength as well as improved room temperature mechanical properties for use in the breeder reactor program ongoing at that time. This alloy as originally developed has been described by Sikka et al., "Production, Fabrication, Properties, and Applications of Ferritic Steels for High-Temperature Applications," 1981, as having the following composition:
C=0.08 to 0.12% PA1 Mn=0.30 to 0.60% PA1 P=0.020% maximum PA1 S=0.0100% maximum PA1 Si=0.20 to 0.50% PA1 Cr=8.00 to 9.50% PA1 Mo=0.85 to 1.05% PA1 V=0.16 to 0.25% PA1 Cb=0.06 to 0.10% PA1 N=0.030 to 0.070% PA1 Ni=0.40% maximum PA1 Fe=Balance PA1 1. A FEM stress analysis of the rotor at steady-state temperature and normal operating speed. Usually the life criteria can be satisfied with a linear elastic FEM solution. For example, if review of the stress analysis reveals that any of the three principal stresses, i.e., radial, axial, or tangential or the special relationship therebetween, as set out in the von Mises equivalent stress, are less than the base metal but exceed the weldment design criteria, the placement of the fusion line leading to such results must be revisited and a new situs therefore selected closer to the axis of the rotor whereupon a new FEM model is constructed, loaded, and reviewed until none of the principal stresses, or the von Mises equivalent stress, exceed such design criteria. PA1 2. Graphical and/or numerical representation of the stress rupture data for the rotor base material with sufficient data to establish the statistical minimum curve, supra, resulting from a plot of the log of stress vs. the Larson-Miller parameter (95 percent confidence band). PA1 3. Stress-rupture data for so-called crossweld samples tested to include the base metal, HAZ, and weld metal in the sample. If these data lie above the minimum line, the design proceeds based on base metal minimum properties. If on the other hand, these data fall below the minimum of the base metal, a new "weldment minimum" line is drawn at or below the lowest weldment data and this becomes the new minimum for design purposes.
In the disclosure of Clark et al. in both '519 and '431, supra, the alloy composition disclosed for deposit onto their worn rotor surfaces for buildup could be construed to be a variation of the Oak Ridge alloy, supra, except for its considerable low chromium content. As will be seen from a later more detailed description of the alloy comprising Embodiment Three of the instant invention, the welding filler wire material of this invention may be considered a specific subset of the Oak Ridge modified 9Cr-1Mo, wherein: (1) compositional limits for additional elements are recited, (2) residual combination limits for materials such as arsenic and antimony as well as tin and lead are specified, and (3) a chromium equivalent factor is defined.
Embodiment Five--Fusion Line Placement. As should be appreciated by those skilled in the art, a weldment may be thought of to consist of several portions including the unaffected base metal, the HAZ of the base metal, the fusion line and the weldment filler metal. It should also be appreciated that the HAZ portion of the weldment is by far the most complex area thereof and usually has a coarse-grained portion immediately adjacent to the fusion line, a fine-grained portion adjacent to said coarse-grained portion, intercritically annealed portion, a tempered portion, and then the unaffected base metal. Further, it is realized that each of these regions of the HAZ are altered by the heating effect of the arc and the molten weld puddle inherent in the welding process. As will be discussed in greater detail, infra, there may be effected a soft zone in the grain refined and intercritically annealed regions of the HAZ which can lead to vulnerability of "type 4" cracking if the normal service operation of the welded component is in the high temperature creep range. This particular embodiment of the instant invention addresses a method to choose the location of the weld fusion line so as to avoid vulnerability to stress rupture failure in the soft zone of the HAZ at a premature time. Further, it is an object of this embodiment to choose or establish the optimum location of the fusion line to assure a design margin for future service life in the range of at least 200,000 hours for the zones of the weldment in the immediate proximity of the fusion line. As previously noted, there is a potential conflict between the amount of weld metal to be deposited in the practice of Embodiment One herein and the optimum location of the fusion line. For example, when the rotor configuration requires a weld restoration of the blade attachment region removal of only the damaged material might permit a repair using the smallest amount of weld metal which needs to be added back thereto prior to machining of steeples and the like. However, such a procedure will effectively place the fusion line high on the rotor disc in a location wherein incoming hot steam, or gases exacerbates the problem. On the other hand, by moving the fusion line further away from the worn surfaces and closer to the main body of the rotor so as to seek a lower operating temperature, there is exaggerated the problem of having to add that much more weld metal during buildup which is both costly in terms of time consumed and potentially costly in terms of the more metal that is added, the more the room for error from unwanted inclusions, etc., in the weld puddle. Accordingly, this Embodiment Five of the invention relies on a new short-cut applied to certain stress analysis procedures, including the finite element method (hereinafter FEM). Although there are numerous texts on the FEM method, there does not appear to be a singular teaching wherein the FEM stress analysis is compared with the representation of stress rupture data obtained from the rotor base material at a preferred confidence limit without the need for first obtaining stress rupture data from cross-weld samples tested to include not only the base material supra, but also the HAZ and the weld metal in the sample.